The usefulness of the phenomenon of magnetostrictivity in linear distance or position measuring devices is recognized by the prior art; for example, see Redding, U.S. Pat. No. 4,305,283; McCrea et al., U.S. Pat. No. 4,158,964; Krisst, U.S. Pat. No. 4,071,818; Edwards, U.S. Pat. No. 4,028,619; and Tellerman, U.S. Pat. No. 3,898,555. A magnet surrounding the magnetostrictive wire marks the location to be measured. Such devices can operate with either mechanical or electrical excitation. When a torsional strain propagating along the wire reaches the area of influence of the magnet an electrical signal is generated. Conversely, when an electrical signal propagating along the wire reaches the area of influence of the magnet a torsional strain is generated. Such linear position detectors are utilized as liquid level detectors. The position of the magnet, and hence the liquid level, is determined as a function of the time required for a torsional disturbance to propagate from one end of the wire through the area of influence of the magnet in the case of mechanical excitation or from the position of the magnet to a sensing apparatus located at one end of the wire in the case of electrical excitation.
In the field of liquid level detection, it is often useful to simultaneously measure liquid level and measure liquid temperature at one or more locations. Many liquids change volume with temperature. Thus a measurement based upon level alone would not distinguish between cases where the mass of liquid had changed and where the mass of liquid is the same but the volume has changed due to a temperature change. Tellerman, U.S. Pat. No. 4,726,226 has proposed a combined apparatus for simultaneously detecting liquid level using a magnetostrictive position detecting apparatus and detecting temperature at a plurality of positions within the liquid via temperature sensitive resistors. Tellerman, U.S. Pat. No. 4,726,226, teaches an encoding technique for transmitting both position and temperature information to a remote site using a single pair transmission line. The resistances of the temperature sensitive resistors are measured and these values are used to vary the period of a pulse generator. Position measurements are made at the varying pulse periods of the pulse generator. A composite signal is transmitted on the transmission line in the form of a series of paired pulses. The time between each pair of pulses is a measure of the liquid level. The time between consecutive pairs corresponds to one of the temperature measurements. The sequence of temperature measurements is known to the apparatus receiving the signal via the transmission line, enabling the pulse period to be translated into temperature. Tellerman, U.S. Pat. No 4,726,226, further teaches the use of two precision temperature independent resistors one having a resistance less than the range of the temperature sensitive resistors and one having a resistance greater than this range. These fixed resistances provide fixed pulse periods enabling correction for any component drift.